Molecular properties of bacterial multidrug transporters

Abstract

One of the mechanisms that bacteria utilize to evade the toxic effects of antibiotics is the active extrusion of structurally unrelated drugs from the cell. Both intrinsic and acquired multidrug transporters play an important role in antibiotic resistance of several pathogens, including Neisseria gonorrhoeae, Mycobacterium tuberculosis, Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa, and Vibrio cholerae. Detailed knowledge of the molecular basis of drug recognition and transport by multidrug transport systems is required for the development of new antibiotics that are not extruded or of inhibitors which block the multidrug transporter and allow traditional antibiotics to be effective. This review gives an extensive overview of the currently known multidrug transporters in bacteria. Based on energetics and structural characteristics, the bacterial multidrug transporters can be classified into five distinct families. Functional reconstitution in liposomes of purified multidrug transport proteins from four families revealed that these proteins are capable of mediating the export of structurally unrelated drugs independent of accessory proteins or cytoplasmic components. On the basis of (i) mutations that affect the activity or the substrate specificity of multidrug transporters and (ii) the three-dimensional structure of the drug-binding domain of the regulatory protein BmrR, the substrate-binding site for cationic drugs is predicted to consist of a hydrophobic pocket with a buried negatively charged residue that interacts electrostatically with the positively charged substrate. The aromatic and hydrophobic amino acid residues which form the drug-binding pocket impose restrictions on the shape and size of the substrates. Kinetic analysis of drug transport by multidrug transporters provided evidence that these proteins may contain multiple substrate-binding sites.

FIG. 1

Resistance mechanisms in bacteria comprise (A) drug inactivation, (B) target alteration, (C) prevention of drug influx, and (D) active extrusion of drug from the cell.

FIG. 2

Schematic representation of the two major classes of multidrug transporters. (A) ABC-type multidrug transporters utilize the free energy of ATP hydrolysis to pump drugs out of the cell. (B) Secondary multidrug transporters mediate the extrusion of structurally unrelated drugs in a coupled exchange with protons or sodium ions.

FIG. 3

Structural model for the 12-TMS multidrug transporters of the MFS. The residues constituting the conserved sequence motifs are shaded.

FIG. 4

Structural model for the 14-TMS multidrug transporters of the MFS. The residues constituting the conserved sequence motifs are shaded.

FIG. 5

Structural model for multidrug transporters of the SMR family. The residues constituting the conserved sequence motifs are shaded.

FIG. 6

Structural model for multidrug transporters of the RND family. The residues constituting the conserved sequence motifs are shaded.

FIG. 7

Structural model for multidrug transporters of the ABC superfamily. Shown is the multidrug transporter LmrA, with six transmembrane helices and an NBD, containing the Walker A and B motifs and the ABC signature sequence. In view of the general four-domain organization, it may function as a homodimer. The Walker A and B motifs and the ABC signature sequence are shaded.

FIG. 8

Extrusion of hydrophobic drugs by multidrug transporters. Drugs are expelled from the cytoplasmic leaflet of the membrane to the external medium.